The present invention relates to an ultrasonic linear motor suitable for a driving source in an electronic equipment and a precision machine.
Electronic equipments and precision machines require an actuator which can be mounted in a small space and can effect precise positioning. Particularly when linear movement is required, a linear motor rather than a rotary actuator is preferred because the former does not require a mechanism for converting a direction of motion.
FIGS. 7A to 7E show the operation of an inchworm mechanism conventionally proposed as such a linear motor. The inchworm mechanism is constructed of a shaft 31 and a tubular traveling body 32 axially movably engaged with the shaft 31. The traveling body 32 is composed of three tubular members (piezoelectric actuators) 33, 34 and 35 which are bonded together at respective axial ends by adhesive or the like. The central tubular member 33 is a piezoelectric actuator capable of axially expanding and contracting, and the opposite tubular members 34 and 35 are piezoelectric actuators capable of radially expanding and contracting. In operation, when the traveling body 32 is intended to be moved rightwardly, for example, as viewed in FIG. 7(a), the left tubular member 34 is radially contracted to grasp the shaft 31 under the condition where the central tubular member 33 is axially contracted and the right tubular member 35 is radially expanded (see FIG. 7(b). Then, the central tubular member 33 is axially expanded to thereby rightwardly move the right tubular member 35 (see FIG. 7(c). Then, the right tubular member 35 is radially contracted to grasp the shaft 31, and the left tubular member 34 is expanded to be loosened (see FIG. 7(d). Then, the central tubular member 33 is axially contracted to thereby rightwardly move the left tubular member 34 (see FIG. 7(e). Accordingly, the traveling body 32 can be rightwardly moved by repeating the above operation.
However, in the above-mentioned inchworm mechanism, annular gaps between the shaft 31 and the opposite tubular members 34 and 35 must be precisely controlled, so that a high machining accuracy of the shaft 31 is required. However, it is very hard to machine a long shaft with a high accuracy. Accordingly, a manufacturing cost will become very high, or a distance of movement of the traveling body will be limited.
Furthermore, the shaft 31 is grasped by a compressive stress of the tubular members 34 and 35. Therefore, when the piezoelectric actuators are vibrated at a high frequency such as a resonance frequency of the members, the tubular members 34 and 35 are broken to cause a reduction in efficiency of the motor.
FIG. 6 shows another type linear motor improved in efficiency proposed by the present applicant (Japanese Patent Application No. 63-60714). The prior art linear motor does not require a high machining accuracy, and can move a long distance, utilizing a resonance condition of components. The linear motor is comprised of a traveling member (vibrating member) 24 formed of an elastic material and a pair of piezoelectric elements 26 and 27 bonded by adhesive or the like to a pair of mounting surfaces 25 formed at opposite corners of the traveling member 24. The traveling member 24 has an inverted U-shape formed by a pair of leg portions 21 and 22 arranged in perpendicular relationship to a longitudinal direction of a rail R and by a body portion 23 connecting the leg portions 21 and 22. The leg portions 21 and 22 and the body portion 23 are vibrated by the piezoelectric elements 25 and 27 with the phases of vibration being suitably shifted, so that the traveling member 24 can be moved on the rail R.
However, the above linear motor has the following shortcomings. First, the piezoelectric elements 26 and 27 formed of ceramics are low in moisture resistance. Accordingly, in the case that the linear motor is used in the environment such as a high-humidity environment and a dusty environment, a service life of the piezoelectric elements 26 and 27 will be reduced. Second, as the piezoelectric elements 26 and 27 project from the mounting surfaces 25 of the traveling member 24, there is a possibility that the forward ends of the piezoelectric elements 26 and 27 will collide with an external member such as a wall during traveling of the traveling member 24, causing the breakage of the piezoelectric elements 26 and 27.